Various topologies are known for providing a controlled DC power source. One such topology is a full-bridge current-fed design. In such a design, a DC current source is alternately switched between diagonals of a full-bridge switching network on a primary side of a transformer. By controlling the periods during which each diagonal is conducting, a desired output voltage on the secondary side of the transformer can be obtained. However, limiting stresses on semiconductor switching components of the bridge is an inherent challenge in such a topology. Toward that end, a variety of snubber, clamp and other stress-reducing circuit designs have been proposed. In addition to controlling the conduction and non-conduction of the bridge circuit, the stress-reducing circuits typically require some control mechanism.
One example of a full bridge power converter with an active clamp circuit is described in U.S. Pat. No. 6,038,142. In the described system, an active clamp circuit composed of a capacitor and a switching MOSFET is connected across the DC side of a full-bridge network of switching transistors. The voltage across the switching network is monitored during the switching cycle. When that voltage reaches zero (called a “zero voltage transition” in the '142 patent), the non-conducting switches in the bridge are turned on. Although the described system does, at least in theory, provide a system that may reduce stresses on the switching transistors during operation of the conversion circuit, multiple monitoring circuits are required. In particular, the described system requires monitoring circuitry to monitor output voltage from the converter on the secondary side of the transformer, as well as to monitor voltage across the switching network on the primary side. This can increase complexity and cost of the power converter. Under certain conditions, the system described in the '142 patent may also be sensitive to noise and transients in the switching network, which could affect the sensing of a zero voltage condition, and thus the operation of the system.
In light of the above and other prior art, there remains a need for power conversion circuits that balance reduction of switching component stress and simplicity of control.